A backlight-type light source is used in a liquid-crystal display in order to irradiate a liquid-crystal panel from the back to a front surface thereof. In recent years, in accompaniment to trends toward larger, thinner, lighter, and longer life liquid-crystal displays, and from a viewpoint of improving video properties of such liquid-crystal displays through on/off control, there has been much interest in light-emitting devices that perform surface light emission through an array of light-emitting diodes (LEDs) mounted on a substrate. Extraction of white light from such light-emitting devices is mainly carried out by the following two methods.
The first of these methods involves providing LEDs that emit light of three colors, R, G, and B, and turning on these LEDs simultaneously such that white light is obtained as a composite of light of these three colors. The second of these methods involves, for example, encapsulating a blue LED in a phosphor-containing resin such that white light is obtained through color conversion of blue light. The structure composed by encapsulation of the blue LED in the phosphor-containing resin is referred to as a “white LED.”
However, the first method is expensive because LEDs of three colors, R, G, and B, are required. On the other hand, potting with the phosphor-containing resin in the second method is carried out against an extremely small LED surface area, which makes it difficult to uniformly form the phosphor-containing resin without unevenness.
For this reason, in recent years there has been interest in a third method that can be used in place of the second method and that involves performing color conversion with respect to blue LEDs using a structure in which a phosphor-containing resin is sandwiched between sheet substrates or a phosphor-containing sheet formed by processing a phosphor-containing resin into a sheet shape (for example, refer to PTL 1 and 2).
Phosphors are vulnerable to oxygen and water vapor. For example, although sulfide phosphors such as SrGa2S4:Eu, CaS:Eu, and SrS:Eu are excellent phosphor materials that can be used to reproduce a wide color gamut due to their sharp emission spectra and (Ba,Sr)3SiO5:Eu can be used as an orange light-emitting phosphor material having high brightness, degradation of these phosphors due to water vapor readily occurs in a high temperature and high humidity environment.
Therefore, it is necessary to provide a means for blocking water vapor when using these phosphors. Although it is difficult to adopt these phosphors in white LEDs, in the case of the aforementioned third method, the problem described above can be dealt with by, for example, covering a phosphor layer with a water vapor barrier film. Examples of such methods that have been proposed include a method in which a protective layer of a silicon compound or the like is provided on a phosphor-containing resin (refer to PTL 3) and a method in which a water vapor barrier layer is formed on the surface of a phosphor-containing resin (refer to PTL 4 and 5).
A pouch structure such as illustrated in FIG. 14 may for example be adopted as a strategy for preventing infiltration of moisture at an edge. In the pouch structure, a phosphor layer 123 is sandwiched between a first film 101 and a second film 102, is further sandwiched between a first water vapor barrier film 121 and a second water vapor barrier film 122 disposed outward of the first film 101 and the second film 102, and is sealed by a sealing resin 103. The pouch structure has a large number of layers and cannot be easily manufactured.
Alternatively, a tape sealing structure such as illustrated in FIG. 15 may be adopted. In the tape sealing structure, a phosphor layer 123 is sandwiched between a first water vapor barrier film 121 and a second water vapor barrier film 122, and edges of the first water vapor barrier film 121 and the second water vapor barrier film 122 are sealed by a sealing tape 130. The sealing tape structure has a small number of layers and can be easily manufactured. Examples of materials that can be used as this tape include aluminum foil, which is a good option for use as this tape, and other materials having water vapor permeability of no greater than 1 g/m2/day (40° C. and 90% RH) such as PET (polyethylene terephthalate) vapor deposited on silica.